Correlational analysis and use of multiple regression and discriminant function analysis show that functional associations involving brain languag areas are disrupted in young Down syndrome adults. Down syndrome subjects use similar regions to process language as controls, but to a lesser extent Subjects with Turner syndrome (45,X), including mosaics, had reduced volume of the hippocampus, and impairment of memory and visuospatial abilities. Mosaic Turner syndrome subjects had volumes of left cerebral hemisphere and subcortical nuclei, and metabolism of left middle temporal region and right/left asymmetry ratio in parietal region intermediate between full Turner syndrome subjects and controls. There is a distinctive cognitive profile in fragile X syndrome that is distinct from other forms of mental retardation. An enlarged brain and altered functional interactions among frontal and subcortical brain regions may contribute to the behavioral effects in this disorder; more focal changes in brain structure and metabolism may be related to cognitive changes.